Commercial hydraulic cements are powders customarily prepared from alumina, silica, lime, iron oxide, and magnesia by firing in a kiln. When mixed with water, they form a plastic mass that hardens via chemical combination and by gelation and crystallization. Thus, the conventional hydraulic cement, of which Portland cement is probably the most well-known example, involves the crystallization of powders.
Contrariwise, U.S. Pat. Nos. 3,720,527 and 3,743,525 disclosed that crystallization was not mandatory for the development of strength in hydraulic cement. Rather, certain glass powders would behave like hydraulic cements, i.e., those powders would react with water and subsequently harden to a solid amorphous body.
U.S. Pat. No. 3,720,527 was concerned with the production of hydraulic cements from glass powders consisting essentially, in weight percent, of about 15-85% Na.sub.2 O and/or K.sub.2 O, 10-80% SiO.sub.2, and 3-20% P.sub.2 O.sub.5. No more than 5% total of such compatible metal oxides as MgO, CaO, SrO, BaO, CdO, ZnO, PbO, and B.sub.2 O.sub.3 could be tolerated.
The cements were prepared by melting a batch for a glass having a composition within the operable ranges. The melt was cooled to a glass and the glass comminuted into particles sufficiently minute to pass a 140 mesh and, preferably, a 200 mesh United States Standard Sieve. When the cement was used, water was added thereto in water-to-glass ratios of 0.25-0.50. In general, those hydraulic cements set up within an hour at room temperature.
The addition of water to the glass powder caused a vigorous reaction to occur with the consequent generating of large quantities of heat. This liberation of heat was derived from the very rapid setting reaction demonstrated by the cements. As initially prepared, the cements exhibited relatively poor chemical durability but, when heated to 150.degree.-200.degree. C. to expel absorbed water, the durability thereof was significantly improved.
U.S. Pat. No. 3,743,525 disclosed the production of hydraulic cements from glass powders consisting essentially, in weight percent, of 20-80% SiO.sub.2, 5-40% Na.sub.2 O and/or K.sub.2 O, 5-70% RO, wherein RO consisted of 0-30% MgO, 0-50% CaO, 0-70% SrO, and 0-35% BaO, and 5-15% NaH.sub.2 PO.sub.4 and/or KH.sub.2 PO.sub.4. The incorporation of the H.sub.2 PO.sub.4 -anion into the glass substantially enhanced the reactivity thereof and often improved the compressive strength of the final cement.
Glass powders of the proper composition were prepared which passed a 140 mesh and, preferably, a 200 mesh United States Standard Sieve. The powder was blended into water in water-to-powder ratios of about 0.25-0.50; room temperature or slightly elevated temperatures normally being employed to expedite the reaction. The use of boiling water, however, was avoided.
U.S. Pat. No. 3,498,802 also discussed the preparation of hydraulic cements from glass powders. That patent described the formation of alkali metal silicate glass powders which exhibited thermoplasticity and which, when contacted with water, would set up in like manner to hydraulic cement. In carrying out the process, glass particles passing a 100 mesh United States Standard Sieve and consisting essentially, in weight percent, of 80-94% SiO.sub.2 and 6-20% Na.sub.2 O and/or K.sub.2 O were exposed to a gaseous environment containing at least 50% by weight steam at a pressure of at least one atmosphere and at a temperature of 100.degree.-200.degree. C. The exposure was continued until at least a surface portion of the powders contained up to about 30% by weight of water.
Unfortunately, those three classes of cements suffered from two principal failure modes, viz., creep, i.e., deformation under load or under its own weight, and slow dissolution in water. It would be highly desirable to provide amorphous hydraulic cements which demonstrate high strength, long term durability in contact with water, and essential freedom from creep under full cure.
Potassium silicates are well known to the industrial arts, most frequently being marketed as an aqueous solution or colloidal solution because of their hygroscopic nature. The gelation or polymerization of aqueous alkali metal silicates through the evaporation of water therefrom as, for example, the simple drying of a potassium silicate solution to an amorphous mass, is likewise quite familiar to the art. Nevertheless, such amorphous masses commence crystallizing after a relatively brief period of time and are, of course, highly subject to attack by moisture. Alkali metal silicates have been and are currently employed as binders, particularly in the formation of articles from particles of inorganic materials, e.g., refractory bricks. That utility, however, is conditioned upon a reaction taking place between the alkali metal silicate and the matrix particles to thereby eliminate the intrinsic hygroscopic character of the silicate.
The instant invention is designed to utilize the bonding qualities of potassium silicates as cements, while at the same time providing means for inhibiting the inherent hygroscopic behavior thereof. Accordingly, a primary objective of the present invention is to provide amorphous hydraulic cements which display high strength, long term durability when in contact with moisture, and essential freedom from creep after full cure; creep being defined as deformation under load or under its own weight.